Underwater spark discharge sound-producing system



Nov. 15, 1966 w. A. KEARSLEY ETAL 3,286,226

UNDERWATER SPARK DISCHARGE SOUND-PRODUCING SYSTEM Filed Jan. 18, 1965PLASMA ENERGY F/G. 1. REGlON FIG. 3.

A POWER t1! TRIGGER SUPPLY E i CIRCUIT LIJ L. .1 g g A JOPPXV 0 ZSOKV 37s R ER 20 E 3 F /G. 6 m 4KV SPARKER w 2 m I m 10 1000 WAYNE A. KEARSLEYFREQUENCY (CPS) LOUIS LUCAS, JR.

DONALD E. WATTS 5 INVENTORS (cu UZF Z @Q/ ATTORNEYS United States PatentOfifice 3,286,226 Patented Nov. 15, 1966 3,286,226 UNDERWATER SPARKDISCHARGE SOUND- PRODUCING SYSTEM Wayne A. Kearsley, Chelmsford, LouisLucas, Jr., Cambridge, and Donald E. Watts, Belmont, Mass., assignors toEdgerton, Germeshausen & Grier, Inc., Bedford, Mass., a corporation ofMassachusetts Filed Jan. 18, 1965, Ser. No. 426,242 11 Claims. (Cl.340-12) 7' This invention relates tounderwater sound-producing systemsand more particularly to underwater electroacoustical systems in whichpulses of electrical energy are suddenly discharged into a transducer togenerate sound pulses.

Sound waves have been used extensively in such underwater activities assurveying the ocean floor and sub fioor, tracking moving bodies,navigation and searching for lost wrecks. A number of dilferent soundsources have been used including explosives, propane gas guns, pulsedcrystal transducers, magnetostrictive transducers and electricdischargespark gaps. It is within the field of spark gaps that this inventionresides.

Underwater spark-producing sound sources have a pair 'of spacedelectrodes disposed at the end of a towing cable. Pulses of electricalenergy are discharged across the electrodes and through the watertherebetween. The are discharge thuscreated, generates sound waves whosecharacteristics are a function of the spark gap design and theelectrical energy discharged therein. Such sparking devices may bedivided into two classes which depend upon the location of the groundelectrode. In the merged-type of spark gap, the second or groundelectrode is disposed sutficiently close to the first or high-voltageelectrode, that the plasma bubble formed at the first electrode inresponse to the pulses of electrical energy, will extend to the secondelectrode before it collapses and a direct flow of current through theplasma from the first to the second electrode takes place. In such asystem, a special low-inductance circuit isv required for impedancematching because the plasma itself has such a low resisttime. The secondor unmerged class of sparkers positions 'the ground electrode beyond themaximum expansion region of the plasma bubble. In this class, theelectrical current flowing in the discharge are must pass through thewater outside the bubble in route to the second electrode. A substantialenergy factor is wasted by the current flow through water. This reducedetficiency has been a serious disadvantage in unmerged sparkers.

Other disadvantages which have been common in underwater sparkers ofboth classes include short life, low intensityoutput, and poor frequencycontrol.

It is, therefore, an object of this invention to provide a newspark-discharge underwater sound source which is not subject to theabove-mentioned disadvantages.

Another object is to provide a new and highly-efficient underwater soundsystem which maximizes the output intensity at the most desirablefrequencies.

A further object is to provide a new high-voltage electrode assembly.

Still another object is to provide a system in which a plurality ofspark discharge devices are arranged to amplify the acoustical energyoutput.

Other and further objects will be hereinafter pointed out in the.following specifications and in the appended claims. In summary, ourinvention resides in an underwater sound source of the unmerged sparkertype in which a high-voltage electrode is spaced from a second electrodeby a distance slightly greater than the radius of the plasma bubbleproduced by energization of the first electrode. The high-voltageelectrode itself is a special unit having a central conductive memberand an outer insulator, the

materials and dimensions of which have been selected for optimumperformance and for maximum life. Preferred electrical andconstructional details are hereinafter set forth.

Our invention will be better understood by referring to the attacheddrawings in which:

FIGURE 1 is a perspective view partly in schematic form of a typicalprior art underwater spark discharge device;

FIGURE 2 is a sectional view of a simple embodiment of our invention;

FIGURE 3 is a sectional view of a novel high-voltage electrode assembly;

FIGURE 4 is a perspective view of a preferred embodiment of ourinvention in which plurality of spark discharge devices are arranged inan array; and

FIGURE 5 is a graph of the relative amplitudefrequency curves for pulsesof different voltages and FIGURE 6 is an electrical schematic diagram ofone embodiment of the present invention.

FIGURE 1 shows a prior art spark-discharge device of the unmerged typehaving an elongated high-voltage electrode 10 covered except at one endby insulator 15, which may be an insulative tape or the like. A secondelectrode 157, normally at ground potential, is disposed remote from theexposed end of high-voltage electrode 10. The distance between electrode19 and the exposed end of electrode 10 is predetermined to insure thatthe former is beyond the limits of maximum expansion of the plasmabubble formed about the tip of electrode 10 during discharge to avoidthe impedance matching problem mentioned above. The maximum expansion ofthe plasma bubble may be determined experimentally by high-speedphotographic techniques, an example of which is reported by Edward A.Martin in an article entitled, Experimental Investigation of aHigh-Energy Density, High-Pressure Arc Plasma in the February 1960 issueof the Journal of Applied Physics, volume 31, number 2. By covering thegreater portion of electrode 10 with insulation 15 and leaving only asmall portion exposed to sea water, the current emitted from electrode10 is concentrated at its exposed end thus creating a high currentdensity at the end of the electrode when high energy pulses aredischarged therethrough. Electrodes 10 and 19 are connected across ahigh-energy source, such as illustrated in FIGURE 6, by coaxial cable13. When a high-energy pulse is fed from such a source to electrode 10,a plasma bubble is quickly formed at the exposed end of the electrodeand it very rapidly expands to a maximum size which is a function of theenergy discharged and the configuration of the sparker assembly. Thebubble is formed through ohmic heating of the sea water into steam andit expands very rapidly while the discharge pulse is rising. The size ofthe bubble is a function of the discharge energy and the current densityin the discharge path. No bubble is formed at electrode 19 because thecurrent density there is not great enough to convert the water intosteam. As the discharge energy enlarges the plasma bubble, it approachesa maximum size which is shown by the dotted line in the figure enclosingthe Useful Energy Region. The useful sound waves produced by electricaldischarge through a sparker are created during the initial expansion ofthe plasma bubble. The size of the bubble and the instantaneous velocityof its expansion are important factors in the characteristics of thesound emitted. The discharge energyexpended within this region, creatingthe bubble, contributes to the sound produced, but all the electricalenergy expended outside this region is wasted. The current which flowsthrough the plasma, must also pass through the regions of sea water,outside the bubble, en route to electrode 19. This current flow is shownschematically as lines 11 in the figure. The greater the separationdistance between electrodes and 19, the longer the current path throughthe water and thus, the more energy that is wasted.

FIGURE 2 shows an embodiment of the applicants invention which is simplein structure and highly elficient in operation. A high-voltage electrode10 is centrally disposed within an insulating member with electricalcoaxial connector 13 providing means for energizing electrode 10. Theground electrode is a cage-like element, for example, cylindricalmeshwork with electrode 10 disposed substantial along the axis of thecylinder. The distance separating the mesh of electrode 20 from theexposed end of electrode 10 is the radius of the cylinder and ispredetermined experimentally by high-speed photographic techniques toinsure that it is slightly greater than the Useful Energy Region shownin FIGURE 1, which is the maximum size of the plasma bubble produced bythe discharge of energy through the terminal portion of electrode 10.Electrode 20 must have a high percentage of open area because theprimary purpose of the spark discharge device is to emit acousticalenergy and a closed electrode would greatly attenuate the sound. Themesh-Work of electrode 20 is designed to pass substantially all theacoustical energy generated during the creation of the plasma bubble,and to provide an effective ground electrode just outside the bubble atits maximum expansion. By so positioning electrode 20, current flowsonly a short distance through seat water between the bubble andelectrode 20, thus assuring that most of the discharge energy is used informing the bubble and very little is wasted in the water outside thebubble. Although electrode 20 is shown as a cylindrical cage, it may beconstructed in other configurations. The cylindrical cage is preferredbecause it maintains the electrodes 10 and 20 parallel for advantagesexplained hereinafter.

FIGURE 3 shows a new electrode assembly which has greatly extended thelife of sparker electrodes. The high voltage electrode 10 is anelongated rod of a conductive metal such as, for example, brass. Aninsulator 15 is coated thereupon or molded thereto. Preferably, theinsulative material is a molded neoprene jacket but other high-impactstrength insulators such as, for example, polyurethane, nylon, epoxyresins, and the like may be used as long as they have good bondingproperties.

It is well known that high-voltage electrodes operated in a salt waterenvironment, will erode or disintegrate at a predictable rate controlledby the energy in the discharge and the number of discharges. It has beenpointed out above that it is important to cover all of the highvoltageelectrode except a small surface area in order to concentrate currentflow the small area and thereby produce a high current density to createthe plasma bubble. If the insulation 15 covering electrode 10 is verythin, then it is most probable that it will crack, break and fall 011?because of the high-power discharges and in so distintegrating, anincreasing area of the surface of electrode 10 is exposed therebylowering the current density until it no longer creates a plasma bubble.If on the other hand, insulation 15 is too thick, it will remain inplace as electrode 10 erodes therein. In this case, the length of thedischarge path increases as the exposed end of electrode 10 recedes intoinsulation 15 until the path becomes too great to support the dischargeat which time the device becomes inoperative.

We have discovered that there is a range of thicknesses for insulator 15which avoids both the foregoing problems because the insulationdisintegrates at substantially the same rate that electrode 10 does. Forthe class of insulators mentioned above, we have found that the bestresults are obtained when the thickness of insulator 15 is approximatelyequal to the radius of electrode 10. The insulator may, however, have adiameter within the range of 1 /2 to 3 times the diameter of electrode10, more or less. By so controlling the dimensions, substantiallyuniform disintegration of electrode 10 and insulator 15 is attained,thus prolonging the useful life of the assembly 25, and maintaining theabove exposed small surface area substantially constant asdisintegration proceeds.

A waterproof electrical plug 17 is disposed at one end of electrodeassembly 25 for holding assembly 25 in position and providing electricalconnection thereto. Plug 17 is made from an insulative material andpreferably from the same material as insulator 15, thus permitting theentire assembly 25 to be formed in a single operation by moldinginsulator 15 and plug 17 onto electrode 10. A raised portion 18 isprovided on plug 17 to maintain the assembly securely in its socket.

An important feature of electrode assembly 25 and particularly of plug17 is that these assemblies may be easily removed and replaced in aminimum of time. Previously, sealed joints were employed for thispurpose and it was necessary to cut them open and remove a connectingbolt before a new electrode assembly could be inserted. This was atime-consuming operation which disabled the entire system while it wasperformed.

As an example, an electrode assembly 25 was made using a inch diameterbrass electrode 10, about which a inch diameter neoprene jacket wasmolded. The useful length of the assembly was 12 inches from plug :17 tothe exposed tip. Such an assembly was used in the system shown in FIGURE4 Where 2,000 joules from a 4 kv. source was discharged and the life ofthe electrode was excellent. A number of assemblies so tested producedover 100,000 discharges at this power level.

It has been found that there is an optimum operating voltage for sparkdischarge devices. FIGURE 5 shows a graph of two curves indicating thefrequency response from discharges of capacitors charged to differentvoltage levels. Curve A represents a 4 kv. pulse and it may be notedthat the fiat top of the curve is found in the lowfrequency region inthe vicinity of c.p.s. which has been found to be the most desirableoperating frequency for underwater seismic surveying. At 100 c.p.s.greater penetration of the ocean bottom is realized, thus, increasingthe total results from seismic surveying. Curve B in FIGURE 5demonstrates a higher acoustical output from pulses of 10 to 20 kv. Thisincreased acoustical output is shifted into the higher frequency rangesthan that produced by a 4 kv. discharge, indicated as curve A. Althoughcurve B demonstrates greater relative amplitude in the output, the shiftof the curve to the undesirable higher frequencies more han offsets theincrease in am plitude. The optimum operating conditions for a sparkdischarge device is, therefore, a pulse in the 4 kv. range from ahigh-capacity energy-storage capacitor.

A power supply suitable for operating the system of FIGURE 2, or one ofthe assemblies 25 in FIGURE 4, is illustrated in FIGURE 2 of US. LettersPatent 2,478,- 906, issued on August l6, 1949 to H. E. Edgerton and nowassigned to the assignee of the present invention. The circuit of FIGURE2, suitably modified, is illustrated schematically in FIGURE 6. Powersupply 31 is adapted to charge capacitor 32 to a high voltage potentialsuch as the above discussed 4 kv. Capacitor 32 is connected in seriescircuit with main electrodes 34 and 35 of triggered spark gap 37 whichhas trigger electrode 36 and with electrodes v10 and 20 of an assemblysuch as illustrated in FIGURE 2. Trigger circuit 33 connects acrosstrigger electrode 36 and main electrode 34 of triggered spark gap 37.When trigger circuit 33 is operated a sufficiently high voltage isimpressed across mai-n electrode 34 and trigger electrode 36 to causethe gas in triggered spark gap 37 to ionize. The high voltage potentialon capacitor 32 is sufiiciently high to cause break-down to occurbetween main electrodes 34 and 35 and capacitor 32 then dischargesbetween electrodes 10 and 20, cansing a plasma bubble to form andgenerating an acoustical pulse. It will be clear to those skilled in theart that a single trigger circuit 33 can be utilized to discharge anumber of charge capacitors 32 into their associated transducers.

Due to the fact that the most desirable acoustical signal is generatedfrom approximately a 4 kv. pulse, the means for increasing the amplitudeof the output signal in the frequency range desired is not increasedvoltage but rather the simultaneous firing of a plurality of suchdevices. An apparatus embodying this principle is shown in FIGURE 4.Three electrode assemblies 25 are linearly positioned equidistant fromparallel metallic supports 20 which act as the ground electrode in thedischarge system. The supporting structure is shown as a triangularframe whose members are an electrically conductive metal such asstainless steel. Cross members 27 provide structural strength and arms28 welded to the cross members (27 clamp the electrical socket 30 foreach electrode assembly 25 securely in place. Each electrode assembly 25is connected by a different cable -13 to a single cable 29 and in turnto sources of energy, usually capacitors, through a common switch, thusproviding simultaneous firing of the electrode assemblies 25.

In addition to triangular frames, other configurations may also be used,such as the cylindrical cage shown in FIGURE 2, rectilinear shapes, etc.The important consideration is that the side members 20 be disposedparallel to the electrode assembly 25 and the minimum spacing beslightly greater than the maximum expansion of the bubble.

For the purposes pointed out above, the electrode assemblies 25 and thetriangular support structure 20 are maintained parallel and electrode issubstantially equidistant from the structural members 20. As the systemis operated, the high-voltage electrode 10 and the insulator will erodeat a substantially constant rate. In so doing, the parallel structuremaintains the predetermined distance which is important for theoperation of the system as pointed out above.

A four-wire cable 29, with one Wire 13 for each electrode assembly and acommon ground lead, is covered with neoprene to provide both theelectrical energy and the towing means to the system shown in FIGURE 4.The plug-in electrode assemblies 25 may be removed and replaced in lessthan a minute which is a great advantage in minimizing the time lost inthis operation. In our preferred embodiment, we have shown threeelectrode assemblies 25 but more or less may be used depending upon thetask to be performed. This three electrode array is a convenient sizewhich is easily handled and yet produces high amplitude sound pulses inthe optimum frequency region. Such an array, constructed of stainlesssteel may be 79 inches long and 10 inches on each side with a totalweight of 28 pounds. Input energy may vary from 500 to 7500 joules perpulse and it may be fired as fast as the electrical system can berecharged and triggered, usually about 4 pulses per second at 500joules.

Although we have shown and discussed our invention in terms of itspreferred embodiment, it has considerably greater scope as will beobvious to those skilled in this art and to whom all such modificationswill be apparent and all such are deemed to fall within the spirit andscope of our invention.

We claim:

1. An underwater sound-producing system comprising:

an insulated electrode having a small area exposed to the water;

an uninsulated electrode disposed a predetermined distance from saidinsulated electrode; and

high-voltage potential means connected across said electrodes andadapted to discharge from the small exposed area of said insulatedelectrode to said uninsulated electrode, said discharge causing a plasmabubble to form about said exposed area, said pre- 6 determined distancebeing just greater than the maximum expansion of said plasma bubble.

2. An underwater sound-producing system as in claim 1 in which saiduninsulated electrode is a sound-passing structure disposed saidpredetermined distance from said exposed area.

3. An underwater sound-producing system as in claim 2 in which saidinsulated electrode comprises an elongated, conductive metal rod, saidmetal rod and insulation being designed to have the same rate ofdisintegration during operation so as to maintain substantially constantsaid small exposed area.

4. An underwater sound-producing system comprising:

an uninsulated electrode;

an insulated electrode having a small area exposed to the water anddisposed a predetermined distance from said uninsulated electrode, saidelectrode and its insulation being designed to have the same rate ofdisintegration during operation so as to maintain substantially constantsaid small exposed area;

supporting means maintaining said electrodes at said predetermineddistance; and

high-voltage potential means including a capacitor charge to a highvoltage connected to said electrodes and adapted to discharge saidcapacitor through the water between said electrodes to form a plasmabubble therebetween, said predetermined distance being just greater thanthe maximum expansion of said plasma bubble.

5. An underwater sound-producing system comprising:

an uninsulated electrode having a plurality of parallel, conductive,rod-shaped members disposed an equal distance from a central axis;

a rod-shaped electrode disposed along said central axis and so insulatedas to present a small exposed area at one end to the water, saidelectrode and insulation being designed to have the same rate ofdisintegration during operation so as to maintain substantially constantsaid small exposed area;

supporting means maintaining said electrodes at said equal distance; and

high-voltage potential means connected to said electrodes and adapted todischarge a capacitor charged to a high voltage through the waterbetween said electrodes forming a plasma bubble therebetween andproducing an acoustical pulse, said equal distance being just greaterthan the maxi-mum radius of said plasma bubble.

6. An underwater sound-producing system as in claim 5 in which saidcapacitor is charged to a high voltage potential of about 4 kv.

7. An underwater sound-producing system comprising:

an uninsulated electrode comprising a cylindrically shaped wire meshhaving a central axis;

a rod-shaped electrode disposed along said central axis and so insulatedas to present a small exposed area at one end to the water, saidelectrode and insulation being designed to have the same rate ofdisintegration during operation so as to maintain substantially constantsaid small exposed area;

supporting means for maintaining said electrodes fixed in position withrespect to each other; and

high-voltage potential means connected to said electrodes and adapted todischarge a capacitor charged to a high voltage potential through thewater between said electrodes forming a plasma bubble therebetween andproducing an acoustical pulse.

8. An underwater sound-producing system as in claim 7 in which the innerdiameter of said uninsulated electrode is just greater than the maximumexpansion of said plasma bubble.

9. An underwater sound-producing system comprising:

a plurality of insulation-covered electrodes each having a small areaexposed to the water;

an uninsulated electrode disposed a predetermined distance from saidelectrodes; and

high-voltage potential means connected across said uninsulated electrodeto each of said insulated electrodes and adapted to discharge from thesmall exposed area of each of said insulated electrodes to saiduninsulated electrode, said discharge causing a plasma bubble to formabout each said exposed area, said predetermined distance being justgreater than the maximum expansion of said plasma bubbles.

10. An underwater sound-producing system comprising:

a plurality of elongated, insulation-covered electrodes each having asmall area exposed to the Water;

an uninsulated electrode frame;

means for mounting said insulation-covered electrodes Within but spaceda predetermined distance from said frame; and

high-voltage potential means connected across said uninsulated electrodeframe to each of said insulationcovered electrodes and adapted todischarge from the small exposed area of each of said insulationcoveredelectrodes to said uninsulated electrode frame, said discharge causing aplasma bubble to form about each said exposed area, said predeter- 0mined distance being just greater than the maximum expansion of saidplasma bubbles.

11. An underwater sound-producing system as in claim 10 in Which saidhigh-voltage potential means includes a high-capacity, energy-storagecapacitor charged to a high-voltage potential of about 4 kv.

References Cited by the Examiner UNITED STATES PATENTS 1,152,697 9/1915Bodde 340-12 1,758,993 5/1930 Wolff 340-12 3,007,133 10/1961 Padberg34012 OTHER REFERENCES CHESTER L. JUSTUS, Primary Examiner.

G. M. FISHER, Assistant Examiner.

1. AN UNDERWATER SOUND-PRODUCING SYSTEM COMPRISING: AN INSULATEDELECTRODE HAVING A SMALL AREA EXPOSED TO THE WATER; AN UNINSULATEDELECTRODE DISPOSED A PREDETERMINED DISTANCE FROM SAID INSULATEDELECTRODE; AND HIGH-VOLTAGE POTENTIAL MEANS CONNECTED ACROSS SAIDELECTRODES AND ADAPTED TO DISCHARGE FROM THE SMALL EXPOSED AREA OF SAIDINSULATED ELECTRODE TO SAID UNINSULATED ELECTRODE, SAID DISCHARGECAUSING A PLASMA BUBBLE TO FORM ABOUT SAID EXPOSED AREA, SAID PREDETERMINED DISTANCE BEING JUST GREATER THAN THE MAXIMUM EXPANSION OFSAID PLASMA BUBBLE.